Peracetic acid (PAA) is a strong disinfectant with a wide spectrum of antimicrobial activity. PAA is conventionally prepared by reaction of concentrated acetic acid (AA) and concentrated hydrogen peroxide (HP). Strong, homogeneous acidic catalysts (e.g. 1-20% sulfuric acid) are usually used to catalyze the reaction toward equilibrium. The reactants are supplied to a reactor and are mixed and converted to product mixture within the reactor. These mixtures are prepared in large quantities at a plant and after reaction, placed in storage or shipping containers and allowed to “cure” for several days during which time the mixture approaches and reaches steady state equilibrium. Because these mixtures are stored and shipped after the PAA formation reaction has reached equilibrium, they are referred to as “equilibrium mixtures”.
The equilibrium PAA mixtures are typically prepared in concentrations between 5-35% (wt) PAA containing excess HP and AA with water making up the balance, i.e., high concentrations of HP and/or AA relative to PAA concentration. Stabilizers must be added to the equilibrium PAA to prevent decomposition during storage and transport to end-users. Major uses of equilibrium PAA include disinfection, bleaching and chemical synthesis. Current practice for such applications is distribution of bulk equilibrium PAA solutions from large manufacturing plants to locations of end-use, often involving multiple distributors and transport events. These solutions must be shipped in compliance with regulations for hazardous materials (corrosive, oxidizer) and are explosive. After delivery to the end-user, the equilibrium PAA is typically stored in vented drums until use. PAA concentrations up to 15% are typically used for water treatment, for sanitizing, disinfecting, and sterilizing in the food and beverage industry, in laundries and for medical applications. Higher PAA concentrations up to 40% are exclusively used for oxidation reactions.
In aqueous solution peracetic acid is in a chemical equilibrium with acetic acid, hydrogen peroxide and water. This equilibrium is represented in the following Equation (1):

Accordingly, a higher concentration of reactants is required to produce a higher concentration of peracetic acid. Conversely, a higher concentration of water will drive the reaction backwards, which means dilute solutions have very low PAA equilibrium concentrations and mostly contain water and unused starting materials.
The molar concentration ratio of products versus reactants gives an equilibrium ratio often referred to as the equilibrium constant. Equilibrium constants for solutions of peroxycarboxylic acids can be determined by common methods. The equilibrium constant of a peroxycarboxylic acid can be determined by the following Equation (2A):
                                                        [                              RCO                ⁢                                  —                                ⁢                O                ⁢                                  —                                ⁢                OH                            ]                        ⨯                          [                                                H                  2                                ⁢                O                            ]                                                          [                              RCO                ⁢                —                ⁢                OH                            ]                        ⨯                          [                                                H                  2                                ⁢                                  O                  2                                            ]                                      =                  equilibrium          ⁢                                          ⁢          constant                                    (                  2          ⁢          A                )            
The equilibrium constant of PAA can be determined by the following Equation (2B):

For equilibrium peracetic acid solutions this equilibrium constant typically ranges between 1.8 and 2.5 (D. Swern, ed., Organic Peroxides, Wiley-Interscience, New York, 1970-72).
An example of typical equilibrium compositions commercially produced and distributed in bulk is 5-35% by weight peracetic acid, up to 30% hydrogen peroxide, up to 40% acetic acid and the balance being water. The weight ratio of hydrogen peroxide to peracetic acid to acetic acid in the merchant products ranges between 4.6:1:1.3 (5-6% PAA equilibrium product) and 1:5.4:6.2 (35% PAA equilibrium product). Using only the [H2O2]:[PAA] ratio is an oversimplified definition for distinguishing equilibrium from nonequilibrium peracetic acid solutions in that it does not represent the acetic acid constituent involved with the equilibrium constant.
There is a large investment cost associated with the production of equilibrium PAA mixtures in a centralized plant, due to the high materials and equipment cost. The extended time needed for reaction to reach equilibrium is a further limitation. Practical production of the equilibrium mixtures requires the use of a catalyst which then needs to be separated from the product by costly purification steps. To minimize the impact of shipping costs, the equilibrium mixtures are produced at relatively high concentrations and then diluted at the point-of-use. However, these mixtures are hazardous and explosive and require costly shipping and handling procedures. The shipping volume is limited to less than 300 gallons per container due to the hazardous nature of the equilibrium mixtures, creating challenging and costly logistics for large volume end-users. The abovementioned issues result in a PAA product mixture that is more costly to the end-user, as well as more dangerous than embodiments of the present invention.
It is possible to produce PAA on-site. Large quantities of equilibrium PAA can be produced by blending concentrated hydrogen peroxide and acetic acid in water. Sulfuric acid may also be added as a catalyst to accelerate the equilibration. The blended solution is allowed to ‘cure’ for at least 6-10 days while reaching chemical equilibrium prior to use. The cure time increases with decreasing concentration of either starting material and is several weeks or longer at very low point-of-use concentrations. Most applications using peracetic acid (with the exception of pulp bleaching) are regulated to use less than 170 mg/L concentrations for hard surface cleaning and less than 80 mg/L for contact with produce and often less than 10 mg/L for water treatment.
As an example of the drawback to producing low concentration equilibrium solutions, a 200 mg/L concentration of peracetic acid in an equilibrium solution would contain 4000 mg/L hydrogen peroxide and 35,000 mg/L acetic acid that is unused starting material (equilibrium constant=2.05). In contrast, nonequilibrium peracetic acid solutions can contain 200 mg/L peracetic acid, 200 mg/L hydrogen peroxide and 160 mg/L acetic acid (equilibrium constant=9315). Therefore to directly produce low concentrations of peracetic acid rapidly and economically on-site, a nonequilibrium product is required.
“Nonequilibrium” refers to chemical mixtures that do not provide a determined equilibrium constant value, such as those determined by Equation (2A) for peroxycarboxylic acids in general, or by Equation (2B) for peracetic acid solutions. Accordingly, a nonequilibrium PAA solution is optionally described as having an equilibrium constant typically as calculated by Equation (2) that is not between 1.8 and 2.5.
Conventional nonequilibrium peracetic acid solutions are commercially produced in bulk by first producing equilibrium PAA, followed by distillation of such equilibrium PAA. The nonequilibrium distillate must then be stored near its freezing point to minimize decomposition and reequilibration during storage. This method of producing nonequilibrium peracetic acid is not practical for on-site end-users due to the complexity of such a production process, the operating skill required, the use of concentrated hazardous materials, and the explosion hazard created by distillation of concentrated peroxides.
To address some of these challenges, there have been various attempts to make solutions of PAA on-site, at the point-of-use. Equilibrium mixtures can be produced on-site by continuous production of a mixture of the individual components of equilibrium PAA (U.S. Pat. No. 6,719,921). The slow rate of reaction to equilibrium requires the use of a strong acid catalyst and therefore the catalyst is present in the product mixture. The reaction of individual components to form the equilibrium occurs in a reaction vessel with enough volume to give the reaction mixture enough residence time to reach equilibrium. This may lead to increased and impractical startup times in the event of planned or unplanned system shutdowns. The nature of the equilibrium mixture means that there is inherently some proportional quantity of reactants (HP and AA) left in the product mixture. This equilibrium reaction utilizes the feedstocks (HP and AA) in a less efficient manner than the irreversible and rapid reaction to produce nonequilibrium mixtures in embodiments of the present invention. The rapid reaction in embodiments of the present invention minimizes the system startup time making it more suitable for on-demand production of PAA at the point-of-use.
Reactive precursor mixtures can be reacted with a stream of alkali metal hydroxide to produce nonequilibrium PAA mixtures at the point-of-use. U.S. Pat. Appln. Pub. No. 20120245228 describes a premixed stream of acetyl donor and hydrogen peroxide reacted with a stream of sodium or potassium hydroxide. The alkaline environment allows for the perhydrolysis reaction of peroxide, producing nonequilibrium PAA mixtures. This process is difficult to control due to the instability of the reactive precursor mixture, as well as the heterogeneity of the precursor mixture, and is less efficient (in terms of % yield) compared to embodiments of the present invention. The lower yield results in a PAA composition with increased acetic acid compared to embodiments of the present invention. The costs associated with preparing the precursor mixture as well as the lower PAA yield for the reaction result in a more costly PAA mixture than embodiments of the present invention.
U.S. Pat. No. 5,505,740 describes a method for in-situ formation of peroxyacid using peracid precursor, a source of hydrogen peroxide and a source for delayed release of acid for a bleaching product (wash solution) and a method of removing soil from fabrics. In the method of Kong et al. the aqueous wash solution is initially raised to a relatively high pH level (e.g., 9.5) to enhance production of the peroxyacid in the aqueous solution, followed by lowering the pH of the aqueous solution by, for example, the delayed release of acid, to enhance bleach performance. The source of the delayed release of acid may be an acid of delayed solubility, an acid coated with a low solubility agent or an acid generating species, or an acid independent of the bleaching product employed.
British Pat. Pub. No. GB 1,456,592 relates to a bleaching composition having both encapsulated bleaching granules and agglomerated pH-adjustment granules acid. The bleaching granules comprise an organic peroxy acid compound stabilized by salt(s) of strong acids and water of hydration, encapsulated in a fatty alcohol coating. The pH-adjustment granules comprise a water-soluble alkaline buffer yielding pH 7-9 agglomerated with a suitable adhesive material to yield the desired solubility delay. Preferred peroxy acid compounds are diperisophthalic acid, diperazelaic acid, diperadipic acid, monoperoxyisophthalic acid, monosodium salt of diperoxyterephthalic acid, 4-chlorodiperoxyphthalic acid, p-nitroperoxy benzoic acid, and m-ehloroperoxy benzoic acid.
U.S. Pat. No. 6,569,286 and published PCT Pub. No. WO 0019006 (App. No. WO1999GB03178) relate to a process for bleaching of wood and non-wood pulp. In this process an agglomerate containing, among others, a bleach activator (e.g., tetraacetylethylenediamine, TAED) and a peroxide soluble binder (e.g., polyvinyl alcohol) is added to a dilute solution of hydrogen peroxide. The components are allowed to react and the pH of the resulting mixture is chemically adjusted to a suitable alkaline pH and the pulp is contacted with the resulting solution.
Peracids can be produced in electrochemical cells or reactors by establishing a potential difference across electrodes immersed in electrically-conducting fluid and introducing appropriate reactant materials. For example, U.S. Pat. No. 6,387,238 relates to a method for preparing an antimicrobial solution containing peracetic acid in which hydrogen peroxide or peroxide ions are formed electrolytically and the hydrogen peroxide or peroxide ions are then reacted with an acetyl donor to form peracetic acid.
U.S. Pat. No. 6,949,178 discloses a process and apparatus for the preparation of peroxyacetic acid at the cathode of an electrolytic cell having an ionically conducting membrane in intimate contact between an anode and a gas diffusion cathode. The method comprises supplying an aqueous organic acid solution to the anode, supplying a source of oxygen to the cathode, and generating peroxyacid at the cathode.
European Patent EP1469102 discloses the process and apparatus for electrolytically producing peracetic acid from acetic acid or acetate using an electrolytic cell incorporating a gas diffusion electrode in the presence of a solid acid catalyst.
JP-T-2003-506120 discloses the electrolytic synthesis of peroxyacetic acid. In this method, oxygen gas is electrolyzed to obtain peroxide species which are then reacted with acetylsalicylic acid to obtain the peroxyacetic acid.
Other disadvantages of known methods are, among others, (1) the long reaction or cure times required to produce equilibrium concentrations of peracetic acid solutions; (2) costs of shipping, handling, and storage, (3) limited shelf life of concentrated acids, bases, and peroxides, which are all corrosives and hazardous materials; (4) cost of shipping large quantities of water containing merchant hydrogen peroxide; (5) the presence of stabilizers or contaminants originating from merchant hydrogen peroxide; and (6) relatively low production rates or excessive equipment size and cost. In addition, the practice of combining bulk chemical constituents obtained from merchant suppliers to produce nonequilibrium peroxycarboxylic acid solutions, including peracetic acid, does not produce the compositions provided herein. Processes and related devices provided herein eliminate these disadvantages and other disadvantages associated with shipping, storing and handling concentrated merchant peracetic acid.
A process that mixes reactants with fewer storage or shipping requirements than the product solution, rapidly and safely, to provide the benefits of a nonequilibrium solution of a peroxycarboxylic acid and the benefits of on-site mixing with a high yield would be advantageous. The process to efficiently produce continuous nonequilibrium PAA requires manipulating and reacting the feedstocks in a particular sequence and maintaining specific ratios to prevent the accidental formation of unsafe mixtures and to maintain proportional flow of feedstock reactants to ensure optimal reaction conversion, and thereby, economic PAA production.
Consequently, there is a need for an efficient process of preparing peroxycarboxylic acids, including peracetic acid, on-site, on-demand, and cost-effectively. Other objectives may appear herein.